M813A1 Transmission Air leak

Working on a truck for someone else, although I did just actually sort of score my own:

Loud air leak in an M813. Only happens in neutral and forward, not reverse. Upon pulling the trans doghouse in the cab, there is what appears to be a female threaded plug fitting with nothing there. I tried searching and the TMs and didn't find much except that air is vented when reverse and first/forward are shifted in and out of, respectively.

I'm throughly stumped and I'm even stumping people like 98G with this one. Any ideas?

Thanks!!!

71DeuceAK said:

Working on a truck for someone else, although I did just actually sort of score my own:

Loud air leak in an M813. Only happens in neutral and forward, not reverse. Upon pulling the trans doghouse in the cab, there is what appears to be a female threaded plug fitting with nothing there. I tried searching and the TMs and didn't find much except that air is vented when reverse and first/forward are shifted in and out of, respectively.

I'm throughly stumped and I'm even stumping people like 98G with this one. Any ideas?

Thanks!!!

Click to expand...

Search through 73M819's posts he has posted quite a bit on 5 ton transfer case air shift failures.

Probably be a good idea to post a picture of the fitting.

My 813 had a similar leak; After a 10 minute drive or so and wjhen going to park in 1st or in neutral I could hear it, but if I cycled into reverse and back to neutral it would stop. The air was venting out of the transfer case breather vent. after driving it around for a while and working the sprag from reverse to 1st a few times the problem eventually went away on its own. Maybe this is the same situation...

Interesting. Maybe just go out there for an hour and shift and shift and shift and shift and it might go away on its own?

Would actually driving it (at slow speed, on private property) around a bunch make any difference as opposed to firing her up and shifting a couple dozen times?

The sPrague changes direction as it goes in and out of reverse. Spraying it full of lubricant and shifting in and out of reverse should d be sufficient.

If it is leaking from the trans air valve air vent (the center teed ports between the two air valves) when out of reverse, the tc air piston is leaking air from the neutral/forward position to the reverse side, then to the trans TC air vent.

The fix is,
1-- put a bunch of air tool oil in the air system, open the air line going to the trans air valves (best done with 0 psi of air), shift in/out of reverse a bunch of times (this is after air is built up to operating psi)
2-- pull the air TC shifting piston, clean, install new O-ring, lube lightly with air tool grease, this can be done with the TC installed and even most times with the front driveshaft/drive flange installed, though it is EASIER with the front drive shaft dropped from the TC, it is a tight fit but can be done.
3--if all else fails, pull the trans poppet air valves, replace/rebuild with NEW O-rings/air tool grease, these valves almost never crap out at most get gunked up because of POOR air system maintenance. All they are is a spring loaded ball that drops into a relief machined into the the reverse shifting rail, other then when the ball is in the relief, the valve is closed.

I tried the spray lube and working it back and forth, is it worth doing this more, i.e sit out there for an hour, first?

10 bucks says it is the air cylinder needs to be rebuild on the transfer case. The O rings have a lip on them and are special. I get my rebuilt kit from Memphis Eq. Air up the truck shut it down and put it in reverse and leave it in reverse. Air down the air tanks still in reverse. We do this so you can get the air cylinder off the piston if not it will hit drive line flange. It will be a tight fit but it will come off. Bend over all eight locking tabs ( pain in the butt) and remove the four bolts and cylinder air end cap and you can leave the air line on. Slide the air cylinder off, clean ,hone or replace if needed. If I remember 5/8 deep well in the center of the piston to remove the nut and washer. If the air cylinder piston does come off use plastic strap wrench and unscrew the piston. Clean piston put on new weird O rings same way they came off and lube cylinder. Install the piston in the cylinder and using finger nail to help lip of first O ring go into the cylinder. The second O ring on the piston will slide in easy because of the way the lip is but you will see that . Place new crush copper ring on transfer case and install the cylinder and piston as one unit back on shaft and replace the nut / washer. Replace the outer copper crush ring and end cap with bolts. Do not bend over the locking tabs till you try and see if you got any air leaks on the copper crush washers. If good bend over the tabs.
Since you have the sheet metal off double check all eight bolts holding on the poppet valve. Two on each valve just hold it to the transfer case and two just hold it together but check all.
The rebuild kit I get has a new cylinder, two O rings, two crush washers.

I found some extra weird rubber seals O rings if we must and two new crush washers. Just hope you cylinder is not pitted to bad as I don't have an extra or I would include it too. You can see they have a lip to catch air pressure. If you want to have them free PM me you name and address.
PS I also found the sprag shaft to transfer case seal. It is a little harder to replace. On the transfer case face and shaft you will see a washer with maybe two punch marks to hold the washer in place. Take a drill bit just a tad bigger then the punch mark and drill very slow just enough to eat the part that is bent into the case. When you remove the washer you will see the seal so replace /lube it and put the washer back and peen/ center punch it to hold it in place then do your piston that is ready to install.

We made another discovery, which perplexes me a little more. We had to use the truck today, of course only at low speeds on private property only, NOT on public roads) and we noted the intensity of the leak varies it seems. In other words, this time it built to as much as 90PSI and wasn't deafeningly loud, though other times after (shifting in/out of gear) it would be fairly loud. It never went below 60PSI though.

Just rebuild the sprag air cylinder and don't over think it.

Roger that! I won't get around to fixing it this summer, he will at this rate as I am leaving next week, but I'm just still trying to wrap my hard head around what is going on inside there; its principle of operation.

Most likely the lip of the forward sprag preformed packing has folded over because of rust on the cylinder walls. Working in back and forth might cause it to catch once and awhile and sound different or less air or more air. It still needs to be rebuilt no matter what. See here is the lip that catches the air and expands out to hold pressure pushing the piston one way or another. Both lips face away from each other on the piston and the smooth side never sees pressure. I now pronounce you expert on the sprag cylinder workings.